Solver Manager: realtime planning with overcommit

SolverManager supports solving more problems than is the number of available CPU cores. In the current implementation, problems coming later have to wait until their predecessors are solved.
With real-time planning, Solvers may run in a daemon mode, which means they never finish. However, in such a daemon mode, the CPU can tan be distributed among all solvers.
This mode could be a new SolverManager config and would turn on the daemon mode on all solvers automatically.
A similar concept of sharing CPU cores in a round-robin manner is already used in Partition search.